Today it is a frequent occurrence, in particular in view of obtaining savings in terms of bandwidth, to multiplex different digital streams within the same signal.
Such multiplexing can be predetermined and its structure is therefore constant over time.
Multiplexing can also be dynamic; that is, that the structure of the multiplexed signal changes over time. In this case, a configuration table, which is part of the multiplex signalling, is generally defined. It can be transmitted by the same transmission channel as the multiplex (called “in-band” signalling) or any other means, for example, by a specific channel, called a signalling channel, which is distinct from the data channel through which the desired digital streams are transported (called “out-of-band” signalling.)
Thus, within the framework of the MPEG4 standard, a configuration table (or allocation table) called FlexMuxChannelTable indicates to which digital stream a specific stream unit is associated, based on its index. Such an index, written in a specific field of the stream unit, is encoded over 256 bits. For example, for a given configuration table, index 112 can correspond to an audio stream, and index 118 to a video stream.
When a stream unit is received, the receiver queries the FlexMuxChannelTable configuration table and determines, according to the stream unit index, the processing that will be applied to it: in the previous example, the FlexMuxChannelTable table indicates to the receiver that the stream unit of index 118 must be transmitted to a video decoder.
Within the framework of applications that operate in “multicast” mode, such an allocation table is sent periodically (for example, once per second within the framework of Internet type applications,) so that it can be quickly available for a user that has just connected.
In the framework of dynamic multiplexing, a new allocation table is sent whenever a change takes place in the multiplex configuration. The receiver uses at each instance the last configuration table received to determine the processing to apply to a specific stream unit.
Nevertheless, this method has the inconvenience of not being very robust. Indeed, if loss or an error occurs during the allocation table transmission, the processing performed by the receiver is incorrect. Using again the example given above within the framework of MPEG4, the receiver can thus switch the stream unit of index 118 to a video decoder, whilst the new configuration table, which it has not received due to a network disturbance, shows that index 188, corresponds to a scene description.
Such losses or deterioration of bursts is frequent, in particular within the framework of network transmissions, which are subject to disturbances.